Apparatus for the manufacture of urea, formed of aluminum bronze



APPARATUS FOR THE MANUFACTURE OF UREA, FGRWED OF ALUli IINUM BRONZE JeanLeon Maurice. Frejacques, Paris, France, assiguor to Pechiney, Compagniede Protluits Chiiniques et Electrometallurgiqnes, Paris, France, acorporation of France Application December 7, 1953, SerialNo. 396,729.

Claims priority, application France December 13, 1952 4 Claims. (6].75-161) No Drawing.

The present invention, which is based upon the results of applicantsresearches, relates to materials employed in the construction ofapparatus used in the synthetic manufacture of urea.

It. is known that one of the difficulties encountered in the industrialsynthesis of urea from CO2 and NH3, carried out at temperatures above130 C., resides in the corrosion of apparatus by the reaction medium.Lead, which is commonly employed, is suitable for use as lining only,because its mechanical properties render impossible its use in the.construction of the apparatus themselves, or of the valves, pipes, etc.Special nickel and/or chromium containing steels, as well as otheralloysgenerally known as being corrosion-resistant, do not function in asatisfactory manner in the presence of the reaction medium underconsideration here. Further, the costly use of silver does not yieldresults in proportion to the price of this metal.

The present invention makes it possible to carry out the production. of.urea with only insignificant corrosion, by the use of certain alloys ofcopper with aluminum.

Accordingto a first feature of the invention, the parts of apparatus,which are subjected to the attack by hot mixtures of urea-ammoniumcarbamate-water, are made of or are. protected with. alloys of specialcomposition belonging to the so-called aluminum bronze group-of alloys.By this is meant copper alloys having 3 to 11% aluminum and, eventually,slight amounts of other metals such,v as Fe, Mn, Ni, Si, V, Cr, Pb, Ti,etc., the sum of which does not exceed 5%.

It is known that these bronzes resist reasonably well corrosion in acidmedia and atmospheric oxidation, but resist poorly corrosion in alkalinemedia, especially, at-a high temperature. Considering the fact thatcopper corrodes most readily in an ammoniacal medium, it was a 6 "ice 21 most unexpected discovery on the part of, applicant/that aluminumbronzes possess high corrosion resistance in the hot basic (ammoniacal)mixtures which urea forms with CO2, NH3 and water during its synthesis.

Since aluminum bronzes possessgood, mechanical characteristics, and areeasily workable, they are used, according to the present invention, inthe construction of the apparatus themselves, such as autoclaves, inletpipes for CO and NH3, or any other pipes or parts terminatingv attheautoclave or situated inside thereof, as well as in the manufacture ofprotecting linings for walls made or other metals.

However, not all aluminum bronzes behave in the; same manner withrespect to the corrosive medium. Applicant has, therefore, as a resultof his researches, discovered and selected those which possess thehighest resistance to corrosion. Hence, according to another'feature ofthe present invention, the apparatus or parts thereof used in thesynthesis of urea are madewith' the view of enhancing their resistanceto corrosion-of aluminum bronze which contains manganese, and whichhasan iron content lower than that of themanganese. The bronzeaccordiugto the invention should contain 0.1 to 4% Mn, while its iron contentshould not exceed that of the manganese.

Particularly satisfactory results are obtained when the alloy iscomposed, in addition to copper, of 8 to 10%- A1, 0.2 to 1% Mn, 0.02 to0.4% Fe and less than 0.3% other elements, while the ratio- Percent FePercent Mn does not exceed 0.5. Preferably, said other elements shouldbe, for example, Cr, V, Ti and/ or, especially, Si.

One of theimportant conditions to the obtainment of as high corrosionresistance'as possible is, that the aluminum bronze should be both poorin iron and that the ratio iron/ manganese below. However, commercialaluminum bronzes generally contain more than 0.5% Feand, most often,even more than 1%.; when-they contain manganese, the content of thiselement is lower than that of Fe. For example, most of commercial alloyshave 1 to 4.25% Fe and those which also contain Mn, have less than 1.5%of the latter as against 2 to. 3.5% Fe. In substantiation oftheforegoing, reference ismade to the followingtable which is reproducedfrom an article entitled: Aluminum Bronze by John L. Everhart,Materials, and Methods, December 1952, vol. 36, #6, pp. 119434,constituting-v Table I found at page 122 thereof.

Table 1.N0minal compositions of aluminum bronzes [The designationsarearbitrary, as explained in the text] 1 Some of these alloys arealsoused in cast form.

I Ni+Mn+Sn 2 a max.

sions.

8 These compost ions are also used in wrought form, particularly foriorgings and extra? Accordingly, the carrying out of the presentinvention into practice under the best conditions required specialaluminum bronzes. For this reason, there have been employed metals poorin iron and care was taken to avoid contamination with iron during thepreparation of .the alloy. In this manner, a bronze was producedpossessing outstanding resistance to corrosion when in contact with hot(150 "200 C.) mixtures of urea, ammonium carbamate and water, underpressure, and practically in the absence of oxygen. The saidbronzereferred to as Special Bronze in the table given below, has thefollowing composition:

Percent Cu 89.80 Al 9.54 Mn 0.45 Fe 0.17 Si 0.04

Apparatus made of this metallic composition have a very much longer lifethan when made of other alloys. In fact, it is found that, whereas theabove identified bronze shows an increase in weight of about 2 gins. persquare metre per annum in a hot urea-carbamate-water mixture, loss inweight amounting to 700 and 730 gms./ sq. mJyear are observed in thecase of aluminum bronzes Ampco 8 and Ampco containing, respectively: 68%Al, 1.53% Fe, and, at most, 0.5 of other elements; and 910% Al,2.75-3.75 Fe, and, at most, 0.5% of other elements.

Alloys having a composition approximating that of applicants outstandingaluminum bronze, the Special Bronze, described above, can also besuccessfully used. This applies especially to bronzes containing PercentAl 9 to 10 Mn 0.2 to 0.6 Fe 0.05 to 0.25

The ratio Percent Fe Percent Mn preferably ranging between 0.1 and 0.45.These alloys may preferably contain 0.01 to 0.1% Si, this silicon beingpartially or completely replaceable by V, Cr or Ti. Alloys within thisrange have a loss by corrosion in the presence of urea amounting to lessthan 100 grs. per square metre per year.

Applicant is aware that it has previously been suggested to use aluminumbronzes in the construction of apparatus intended for use in thesynthetic manufacture of urea. Thus, in French Patent 826,281, issued tothe same assignee as of the present applicationformerly known asCompagnie de Produits Chimiques et Electromtallurgiques Alais, Froges etCamargueit is proposed to use for this "purpose apparatus constructed ofsteel provided with lining of tin or lead, nickel and various alloyshaving a low or high content of aluminum. In a later French patent, viz958,503, likewise issued to applicants assignee, it is stated thatchromium-nickel containing stainless steel, copper alloys such asaluminum bronzes, Monel metal, and copper alloys can be used with urea.Further, in an article by Bolotov in the Russian periodical: ZhurnalKhimicheskoi Promyshlennosti (Journal of Chemical Industry, Moscow),Number 10, May, 1937, there are given the results of corrosion tests in4 the presence of urea, carried out with various metallic substancesand, among others, with B. A. J. M. bronze (Correx Metal) having thefollowing composition:

It is clear from the data set forth in the article that thi aluminumbronze has a resistance to corrosion distinctly less than that of thealuminum bronzes of the present invention. That is, whereas, the bronzesdescribed in the Bolotov article suffered a loss in weight of 1290grammes per square metre per year, applicants aluminum bronze of thecomposition set out above i. e. Special Bronze actually shows anincrease of 2 grammes per square metre per year.

In comparison with the aluminum bronzes proposed in the prior artdiscussed above, applicants aluminum bronzes are characterized by acritical relationship of the proportions of the constituents used,particularly, as regards the actual and relative proportions of Fe andMn, when both of these are used in formulating the alloy. As indicatedearlier in the present specification, applicant has discovered that itis of critical importance that the aluminum bronze-for the purpose inviewhave a very low iron content, that the total Fe and Mn content notexceed 5%, and that the ratio of iron to manganese not exceed 3/4;applicant has also disclosed preferred compositions for his alloys. Whenaluminum bronze is formulated with due regard to the criticalrelationships taught by applicant, then, the resultant product possessesto an exceptional degree resistance to corrosion when in presence of ahot-urea-ammonium carbamatewater mixture.

As disclosed above, the preferred alloys of applicants invention, i. e.those possessing the most outstanding characteristics as regardsresistance to corrosion, contain both Fe and Mn within the proportionsspecified. However, in the course of applicants researches, it wasfurther established that aluminum bronzes which are free of manganese,but which have a very low iron content, also possess valuable propertiesas regards resistance to corrosion. This is particularly true of abronze-identified as bronze A on the table set out belowhaving thefollowing composition:

Percent Cu 90.99

A1 8.96 Fe 0.048

1 By difference.

This alloy shows a loss of 109 grammes per square metre per year in thepresence of urea. Bronzes of which applicants bronze A is one example,belong to the group of aluminum bronzes comprehended within applicantsinvention and which have the following composition:

Metals other than copper, less than 0.3%. Balance copper.

Aluminum bronzes which are free of manganese but contain a small buteffective amount iron, in general show a loss less than 700 grammes persquare metre per year and, hence, are more valuable in this regard thanthe commonly available aluminum bronzesas will be apparent from thefollowing tabulation which compre- 4. Apparatus for use in the synthesisof urea from ammonia and carbon dioxide, characterized in that the Lossof weight calculated in grammes per square metre per year CompositionCorrosion Percent Percent Percent Percent Al Cu Fe Mn (by difierence)Applicants Bronze A 90. 99 0. 048 none 8.96 109 Applicant's Bronze C 88.54 0. 048 9.49 126 Applicant's Special Bronze 89. 8 0.17 0. 45 9.54 (+810.04) +2 AmpcoS 88. 5-921) 1.5-3.0 6-? I(oth(e)ar51)netalsless -700Ampco 86.2-87.7 2. 75-3. 76 9-10 (other metals -730 less than 0.5). B.A. .T. M. (Russia) (Metal Cortex) 84.5 3.2 0.7 11.6 -1,300

(Bolotov).

Norm-Minus sign indicates loss of weight; plus sign indicates gain ofweight.

I claim:

1. Apparatus for use in the synthesis of urea from ammonia and carbondioxide, characterized in that the parts of the apparatus in contactwith the hot reaction medium are formed of aluminum bronze, containingPercent Aluminum 8 to 10 Manganese 0.2 to 1 Iron 0.02 to 0.4Constituents other than copper less than 0.3

Balance copper;

the ratio of iron to manganese being less than 0.5.

2. Apparatus according to claim 1 which includes silicon as one of saidconstituents.

3. Apparatus for use in the synthesis of urea from ammonia and carbondioxide, characterized in that the parts of the apparatus in contactwith the hot reaction medium are formed of aluminum bronze, containingPercent Aluminum 9 to 10 Manganese 0.2 to 0.6 Iron 0.05 to 0.25

Balance copper;

the ratio of iron to manganese ranging from 0.1 to 0.45.

parts of the apparatus in contact with the hot reaction medium areformed of aluminum bronze, which consists of (24th ed., 1940), page1237.

Louis Cassier Co. Ltd., Metals and Alloys 1941 edition), page 13.

1. APPARATUS FOR USE IN THE SYNTHESIS OF UREA FROM AMMONIA AND CARBONDIOXIDE, CHARACTERIZED IN THAT THE PARTS OF THE APPARATUS IN CONTACTWITH THE HOT REACTION MEDIUM ARE FORMED OF ALUMINUM BRONZE, CONTAININGIRON BALANCE COPPER; THE RATIO OF IRION TO MANGANESE BEING LESS THAN O.5